1. Field of the Invention
The present invention relates to a power transmission device of the type wherein the power from a drive shaft is transmitted to a driven shaft via a planetary gear mechanism.
2. Description of the Prior Art
A planetary gear mechanism is generally provided with a ring gear, planetary gears and a sun gear. In order to allow the planetary gears to make a smooth orbital movement, it is necessary that the relative axial movement thereof between the ring gear and sun gear be suppressed to a suitable extent and that the orbit of the planetary gears be kept accurately concentrical with the pitch circles of the ring gear and sun gear. In a conventional planetary gear mechanism, the ring gear, planetary gears and sun gear have the respective gear supporting mechanisms. This causes the structure of the planetary gear mechanism to be complicated. Moreover, the space occupied by the gear supporting mechanisms is increased. Accordingly, the miniaturization and reduction of weight of the planetary gears are restricted to a great extent. In addition, the centering of the gears requires a high-precision processing technique. When the gears make even a slight eccentric rotation, they are not engaged with each other accurately. As a result, noises increase and the gears are worn early.
In order to transmit power from the drive shaft to the planetary gears and interrupt such transmission of power, by a centrifugally operated friction clutch, an inner contactor of the centrifugally operated friction clutch is rotated first by the drive shaft.
The inner contactor generally has on its outer circumferential surface a lining to increase the frictional force between the contact surfaces of the inner contactor and an outer contactor. When the number of revolutions of the drive shaft exceeds a predetermined level, the inner contactor is radially outwardly moved by a centrifugal force to come into frictional contact with the outer contactor and thereby transmit the rotation of the drive shaft to the outer contactor.
The rotation of the outer contactor of the centrifugally operated friction clutch is immediately transmitted to the ring gear in the planetary gear mechanism. When the ring gear is rotated, the planetary gears engaged therewith are orbited around the sun gear as the planetary gears revolve around their own axes. During such a movement of the planetary gears, they transmit only their orbital movements to a driven shaft.
Since the outer contactor of the centrifugally operated friction clutch in a conventional power transmission device and the ring gear in the planetary gear mechanism in the same device are separately constructed, it is necessary that the power transmission device be provided with not only a torque transmission means between the outer contactor and the ring gear but also a support means for separately supporting the outer contactor and ring gear.
The centering of the outer contactor and ring gear requires a high-precision processing technique. When the ring gear in particular is even slightly eccentric, the gears are not engaged with each other accurately. This causes an increase in noises and an early wear on the gears.
Since the outer contactor and ring gear are rotated with the respective bearing portions, an increased inertia force is generated and a great shock occurs due to a sudden operation of the clutch.